It has long been recognized in the field of coin and currency testing that a balance must be struck between the conflicting goals of "acceptance" and "rejection"--perfect acceptance being the ability to correctly identify and accept all genuine items no matter their condition, and perfect rejection being the ability to correctly discriminate and reject all non-genuine items. When testing under ideal conditions, no difficulty arises when trying to separate ideal or perfect coins from slugs or counterfeit coins that have different characteristics even if those differences are relatively slight. Data identifying the characteristics of the ideal coins can be stored and compared with data measured from a coin or slug to be tested. By narrowly defining coin acceptance criteria, valid coins that produce data falling within these criteria can be accepted and slugs that produce data falling outside these criteria can be rejected. A well-known method for coin acceptance and slug rejection is the use of coin acceptance windows to define criteria for the coin acceptance. One example of the use of such windows is described in U.S. Pat. Nos. 3,918,564 and 3,918,565, both assigned to the assignee of the present invention.
Of course, in reality, neither the test conditions nor the coins to be tested are ideal. Windows or other tests must be set up to accept a range of characteristic coin data for worn or damaged genuine coins, and also to compensate for environmental conditions such as extreme heat, extreme cold, humidity and the like. As the acceptance windows or other coin testing criteria are widened or loosened, it becomes more and more likely that a slug or counterfeit coin will be mistakenly accepted as genuine. As test criteria are narrowed or tightened, it becomes more likely that a genuine coin will be rejected.
U.K. Application Serial No. 89/23456.1 filed Oct. 18, 1989, and assigned to the assignee of the present invention, is one response to the real world compromise between achieving adequately high levels of acceptance and rejection at the same time. This U.K. application describes techniques for establishing non-uniform windows that maintain a high level of acceptance while achieving a high level of rejection.
Another prior art approach is found in the Mars Electronics IntelliTrac.TM. Series products. The IntelliTrac.TM. Series products operate substantially as described in European Patent Application EP 0 155 126, which is assigned to the assignee of the present invention.